I connected CD Player to the Tuner input and listened to many CDs over the weekend. There were no problems. However, I noticed, when cabling up, that there was a loose connection with one of the banana plugs connecting the right hand speaker to the amp. Therefore, I fitted a new, more secure, banana plug before listening to CDs via the Tuner input. Although the loss of sound/distortion I had initially experienced came via the left hand speaker - it makes me wonder if the loose banana plug connecting the right hand speaker was the cause of the problem?
Could be, but I doubt it - because you said other sources worked without problem through the speakers.
Now all you can do is connect the CDP back to the CD input......if no further problem = it was your dodgy banana plug.
Same problem on CD = selector switch contacts still in need of cleaning.